Solar energy

The Sun is the source of most of the energy that drives the climate systems on Earth. The nuclear reaction processes occurring within the Sun have caused it to expand over time and gradually become brighter. Models indicate that the earliest Sun shone 25 to 30 percent more faintly than today; its luminosity has slowly increased to what it is today. This scenario has created an intriguing problem for climate scientists, however. A decrease of just a few percentage points of luminosity of the present-day Sun's current strength would cause all the water on Earth to freeze, despite the warming of the natural greenhouse effect. If all the water froze, the resultant high albedo would keep them from melting. Climatic models simulating this weak Sun with greenhouse gases on Earth at their present-day values show that Earth would have been frozen for the first 3 billion years of its existence.

The geologic record of ancient Earth's climate, however, does not show this to be the case. Evidence exists of geologic formations caused by running water, which indicates an unfrozen planet. The first evidence of deposition from glaciers in polar regions does not show up in the geologic record until 2.3 billion years ago. This evidence of an unfrozen Earth is also supported by the continued presence of life on Earth, which is found in the fossil record.

This period is referred to as the Faint Young Sun Paradox. It is still viewed today as a scientific mystery: with so weak a Sun, why was Earth not frozen for the first two-thirds of its history? The mystery is compounded with evidence of Snowball Earth (an Earth completely frozen) from 850 to 550 million years ago, when the Sun was only 5 percent weaker than today (unlike the early Sun, which was 30 percent weaker). In order to explain this, some scientists have suggested that greenhouse gases have acted like a thermostat, regulating the Earth's atmospheric temperature over time.

Today, the Sun produces incredible amounts of energy. It would take 440 million electric power plants to equal the energy coming to the Earth from the Sun. The intensity of the Sun, however, is not constant. Changes that occur on the inside of the Sun can affect the intensity of the sunlight that reaches the Earth's surface. If the intensity of sunlight increases, it causes warming on the Earth; conversely, if solar intensity weakens, it causes cooling on Earth. Scientists at NASA determined that the Sun's intensity weakened from 1500 to 1850. During this time, temperatures on Earth decreased about 2°F (1.2°C). The effect was especially noted in North America and Europe and is known by historians as the Little Ice Age.

The radiation coming from the Sun is short-wave radiation (as opposed to the energy given off by the Earth, which is considered longwave radiation). There are cycles associated with energy output by the Sun. The sunspot cycles are some of the better-understood ones. They occur on roughly an 11-year cycle. Sunspots appear as dark areas and can appear in clusters of only a few to dozens. They last about a week, are caused by magnetic activity, and appear dark because they are cooler than the surrounding areas.

When sunspot activity is high, solar flares occur, which are bursts of high-energy particles. When these occur, the Earth gets bombarded with higher levels of solar radiation. It has been proposed that there is a connection to warmer or cooler climates on Earth being tied in some way to levels of solar activity; that perhaps levels of solar activity act as a trigger for specific climatic trends and conditions. The period from 1645 to 1715, for example, corresponds to the coldest portion of the Little Ice Age, called the Maunder minimum. It has been determined that this time period also had little or no sunspot activity.

When the Sun's radiation energy reaches the Earth's atmosphere, many things can happen to it. It can be reflected, absorbed, or transmitted. The Sun's energy arrives as electromagnetic radiation in the entire spectrum. When it reaches the atmosphere, the ozone layer absorbs the ultraviolet wavelengths (UV).

Ozone is a gas that occurs naturally in the Earth's atmosphere, specifically the layer called the stratosphere, several miles above the Earth's surface. It may exist in only very small amounts, but it serves a critical purpose to life on Earth. It acts as a shield against the Sun's harmful UV radiation that can cause skin cancer. Scientists have discovered that the use of chemicals, such as chlorofluorocarbons (CFCs), are harmful to natural ozone and actually depleted it, allowing an increase in harmful UV radiation to reach Earth. CFCs are organic compounds that contain carbon, chlorine, and fluorine atoms. They are very effective refrigerants that were developed to use in refrigeration units and air conditioners. The most common CFC was marketed under the trade name Freon. Because of their destructive effect on the ozone layer, their use has been banned in the United States.

When the cause of the depletion was discovered, international agreements were put into place to regulate their emissions. Because there has been international cooperation, scientists believe the ozone layer will eventually recover. It is this type of international cooperation that scientists would like to see happen with the global warming issue.

Water vapor and CO2 absorb the infrared portion of the spectrum. Particles in the atmosphere scatter much of the Sun's incoming radiation. Of all the incoming energy, usually only 60 percent ever reaches the Earth's surface. The nature of the Earth's surface then determines what happens to the energy. Some of the energy is directly reflected back into space. The amount reflected by the Earth is determined by the ozone and climate